impl(bigtable): export grpc metrics

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces a GrpcMetricsExporter to export gRPC OpenTelemetry metrics for Google Cloud Bigtable, integrating it into the DataConnectionImpl and adding corresponding build configurations and unit tests. The review feedback highlights several key improvements: enforcing the singleton pattern for GrpcMetricsExporterRegistry by making its constructor private, optimizing latency boundary generation via static caching, removing an unused default parameter in an internal namespace lambda to align with the style guide, and resolving a potential use-after-move/evaluation-order issue when constructing MonitoredResourceResult.

[codecov]

Codecov Report

❌ Patch coverage is 75.35211% with 140 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.23%. Comparing base (0ba8e5f) to head (017e70a).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...d/bigtable/tests/observability_integration_test.cc	12.58%	125 Missing ⚠️
...e/cloud/bigtable/internal/grpc_metrics_exporter.cc	93.58%	10 Missing ⚠️
...d/bigtable/testing/embedded_server_test_fixture.cc	0.00%	3 Missing ⚠️
...ud/bigtable/internal/grpc_metrics_exporter_test.cc	99.10%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16241      +/-   ##
==========================================
- Coverage   92.29%   92.23%   -0.07%     
==========================================
  Files        2221     2225       +4     
  Lines      207373   207960     +587     
==========================================
+ Hits       191391   191804     +413     
- Misses      15982    16156     +174     

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